Potassium channels are involved in a number of physiological processes, including regulation of heartbeat, dilation of arteries, release of insulin, excitability of nerve cells, and regulation of renal electrolyte transport. Potassium channels are thus found in a wide variety of animal cells such as nervous, muscular, glandular, immune, reproductive, and epithelial tissue. These channels allow the flow of potassium in and/or out of the cell under certain conditions. For example, the outward flow of potassium ions upon opening of these channels makes the interior of the cell more negative, counteracting depolarizing voltages applied to the cell. These channels are regulated, e.g., by calcium sensitivity, voltage-gating, second messengers, extracellular ligands, and ATP-sensitivity.
Potassium channels are made by alpha subunits that fall into 8 families, based on predicted structural and functional similarities (Wei et al., Neuropharmacology 35(7):805-829 (1997)). Three of these families (Kv, Eag-related, and KQT, now referred to as KCNQ) share a common motif of six transmembrane domains and are primarily gated by voltage. Two other families, CNG and SKIK, also contain this motif but are gated by cyclic nucleotides and calcium, respectively. The three other families of potassium channel alpha subunits have distinct patterns of transmembrane domains. Slo family potassium channels (also known as BK channels) have seven transmembrane domains (Meera et al., Proc. Natl. Acad. Sci. U.S.A. 94(25):14066-71 (1997)) and are gated by both voltage and calcium or pH (Schreiber et al., J. Biol. Chem. 273:3509-16 (1998)). Another family, the inward rectifier potassium channels (Kir), belong to a structural family containing 2 transmembrane domains (see, e.g., Lagrutta et al., Jpn. Heart. J. 37:651-660 1996)), and an eighth functionally diverse family (TP, or “two-pore”) contains 2 tandem repeats of this inward rectifier motif.
Potassium channels are typically formed by four alpha subunits, and can be homomeric (made of identical alpha subunits) or heteromeric (made of two or more distinct types of alpha subunits). In addition, potassium channels have often been found to contain additional, structurally distinct auxiliary, or beta, subunits (e.g., Kv, Slo, and KCNQ potassium channel families). These beta subunits do not form potassium channels themselves, but instead they act as auxiliary subunits to modify the functional properties of channels formed by alpha subunits. For example, the Kv beta subunits are cytoplasmic and are known to increase the surface expression of Kv channels and/or modify inactivation kinetics of the channel (Heinemann et al., J. Physiol. 493:625-633 (1996); Shi et al., Neuron 16(4):843-852 (1996)). In another example, the KCNQ family beta subunit, minK, primarily changes activation kinetics (Sanguinetti et al., Nature 384:80-83 (1996)).
The Kv superfamily of voltage-gated potassium channels includes both heteromeric and homomeric channels that are typically composed of four subunits, as described above (see, e.g., Salinas et al., J. Biol. Chem. 272:8774-8780 (1997); Salinas et al., J. Biol. Chem. 272:24371-24379 (1997); Post et al., FEBS Letts. 399:177-182 (1996)). Voltage-gated potassium channels have been found in a wide variety of tissues and cell types and are involved in processes such as neuronal integration, cardiac pacemaking, muscle contraction, hormone section, cell volume regulation, lymphocyte differentiation, and cell proliferation (see, e.g., Salinas et al., J. Biol. Chem. 39:24371-24379 (1997)). Some alpha subunits of the Kv superfamily, of which the channels are composed, have been cloned and expressed, e.g., Kv2.1, Kv2.2, Kv5.1, Kv6.1 (Drewe et al., J. Neurosci. 12:538-548 (1992); Post et al., FEBS Letts. 399:177-182 (1996)); Kv8.1 (Hugnot et al., EMBO J. 15:3322-3331 (1996)); and Kv9.1 and 9.2 (Salinas et al., J. Biol. Chem. 39:24371-24379 (1997)). Expression patterns of some of these genes have also been examined (see, e.g., Verma-Kurvari et al., Mol. Brain. Res. 46:54-62 (1997); Maletic-Savatic et al., J. Neurosci. 15:3840-3851 (1995); Du et al., Neurosci. 84:37-48 (1998)).